1. Field of the Invention
The present invention relates to a separator for a lithium ion secondary battery, and relates to a lithium ion secondary battery provided therewith.
This application is based on Japanese Patent Applications Nos. 2002-270620 and 2002-309623, the contents of which are incorporated herein by reference.
2. Description of Related Art
With recent rapid reductions in size, weight, and thickness, various information terminal devices such as note-type personal computers, cellular phones, and video cameras have widely been used. Also, they have begun to be used in hybrid automobiles and fuel cell automobiles.
Under these circumstance, demand for secondary batteries having high energy density as a power source for these automobiles has increased. A lithium ion secondary battery using a non-aqueous electrolyte has high operating voltage and it has already been put to practical use as a battery having high energy density. The lithium ion secondary battery has a structure such that the group of electrodes comprising a positive electrode, a negative electrode and a separator having electric insulating properties and liquid retention properties interposed between the positive electrode and the negative electrode are accommodated in a battery can which is capable of serving as a negative electrode terminal, together with a predetermined non-aqueous electrolytic solution such as an organic electrolytic solution, and the opening portion of the battery can is sealed through a sealing plate provided with a positive electrode terminal via an insulating gasket.
As described above, the lithium ion secondary battery has advantages such as high operating voltage and high energy density, but it has a problem in that leakage of an electrolytic solution is liable to occur because a volatile organic solvent is generally used as the electrolytic solution, and in that a sealing method of the battery was complicated. Also, there is pointed out a possibility of ignition due to overcharging and the use for automobiles is limited. Furthermore, higher energy density and extension of charge and discharge cycle life are strongly required.
Therefore, as the separator for lithium ion secondary battery, a porous film made of a polyolefin resin such as polyethylene or polypropylene, which is excellent in safety, is widely used. The porous film made of the polyolefin resin has so-called shutdown characteristics wherein a porous structure is converted into a non-porous structure due to heat fusion when the interior of the battery is heated, thereby to terminate the reaction between electrodes, thus preventing ignition of the organic solvent, and also has important characteristics to ensure safety of the lithium ion battery. Also, the polyolefin resin is suited for use as a separator material because of low reactivity with the organic solvent.
However, the porous film made of the polyolefin resin had a problem in that pores are merely filled with an electrolytic solution in the case of holding the electrolytic solution, and retention properties of the electrolytic solution are poor because the porous film has poor affinity with the electrolytic solution. Poor retention properties of the electrolytic solution sometimes caused problems such as decrease in capacity of the battery, deterioration of cycle characteristics and limitation of service temperature. Furthermore, the polyolefin resin is likely to form a space at the interface with electrodes because of poor bondability with the other resins or materials, thus causing a decrease in battery capacity and deterioration of charge and discharge characteristics.
To solve the above problems caused when using the porous film made of the polyolefin resin as the separator, use a polyvinylidene fluoride resin in place of the polyolefin resin was also investigated. The polyvinylidene fluoride resin has good affinity with the electrolytic solution and is also excellent in retention properties of the electrolytic solution and adhesion to electrodes.
However, in the case in which the separator made of the polyvinylidene fluoride resin holds the electrolytic solution, a dimensional change may occur due to swelling of the polyvinylidene fluoride resin. Such a dimensional change in the lithium ion secondary battery sometimes causes a problem in that insulating properties between electrodes cannot be maintained.
As the separator whose dimensional change is suppressed, for example, a composite resin film obtained by filling a reinforcing material layer composed of a polyolefin resin nonwoven fabric or polyolefin resin porous film with a polyvinylidene fluoride resin, and a composite resin film obtained by laminating a polyvinylidene fluoride layer with a reinforcing material layer are proposed (see, for example, Japanese Patent Application, First Publication No. 2001-176482).
In these composite resin films, the dimensional change caused by swelling of the polyvinylidene fluoride resin is suppressed, however, since the polyvinylidene fluoride resin has no pores, the polyvinylidene fluoride resin holds the electrolytic solution while uniformly swelling and gelling in the case of holding the electrolytic solution. In a uniformly gelled state, the fluidity of the electrolytic solution drastically deteriorates and the ionic conductivity decreases, resulting in decrease in battery capacity. Furthermore, even when using the porous film as the reinforcing material layer, shutdown characteristics are likely to be obstructed by the polyvinylidene fluoride resin in the case the porous film is filled with the polyvinylidene fluoride resin, thus the use of such a separator is limited.
As the separator, whose shutdown characteristics are not obstructed, having high ionic conductivity, a separator comprising a polyolefin microporous film and a polymer layer scattered on one or both surfaces of the polyolefin microporous film at a surface coating ratio of 50% or less is disclosed (see, for example, Japanese Patent Application. First Publication No. 2001-118558).
In such a separator, shutdown characteristics are not obstructed and high ionic conductivity can be satisfactorily maintained. However, since the surface is not uniformly coated with the polymer layer, ionic conductivity varies locally. When a difference in ionic conductivity arises, transfer of ions increases at the portion with low ionic conductivity, thereby forming local electrode dendrite and causing internal short-circuiting.